Gen Chem 2 help needed please!

[Swenis]

Hi everyone

I'm in the first week of my summer general chemistry 2 class, and I have a few questions about some practice problems that are giving me trouble. I have a quiz tomorrow so I'm trying to get everything ingrained to memory. If you all could help me with these few and explain how, I would be evermost gracious! Thank you all!

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1) Which of the following compounds has the highest boiling point? The highest freezing point? .0250 m C2H4(OH)2 (nonelectrolyte), 0.0200 m NaCl, 0.00750 m MgSO4, 0.0100 m K2SO4, or 0.00400 m Al2(SO4)3.

The answer sheet says that NaCl has the highest boiling point and MgSO4 has the highest freezing point, but I don't know why.

2) The cooling system of a car is filled with a solution formed by mixing equal volumes of water (density = 1g/1mL) and ethylene glycol, C2H6O2 (density = 1.12g/1mL). Estimate the freezing point and boiling point of the solution.

I calculated 109.21 oC for the boiling point and -1.68 oC for the freezing point.
Can anyone verify this?

3) a) A sample of H2 gas is generated in a closed container by reacting 2.050g Zn(s) with 15.0 mL of 1.00M H2SO4. Write the balanced equation for the reaction, and calculate the number of moles of H2 (g) formed, assuming the reaction is complete.

b) The volume over a solution is 122 mL. Calculate the partial pressure of the gas in the solution.

c) The Henry's law constant for hydrogen gas in water at 25 oC is 7.8E-4 mol/L-atm. Estimate the number of moles of hydrogen gas that remain dissolved in the solution. What fraction of the gas molecules is dissolved in solution?

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I'm sorry for all the typing, but I have been working on pages of problems for like six hours, and I don't know these last ones. Any help is very appreciated!

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[ms dagny]

for 3a) there are 0.015 mol of H2 (g)
b) 3.008 atm

I'm not sure about part (c) but hopefully my answers are correct and can help you solve the last part of the question! good luck! =)

 

[Lemmiwinks]

1) Which of the following compounds has the highest boiling point? The highest freezing point? .0250 m C2H4(OH)2 (nonelectrolyte), 0.0200 m NaCl, 0.00750 m MgSO4, 0.0100 m K2SO4, or 0.00400 m Al2(SO4)3.

The answer sheet says that NaCl has the highest boiling point and MgSO4 has the highest freezing point, but I don't know why.

This has to do with the colligative properties of solutions. The more moles of stuff dissolved in water, the higher the solution's boiling point and lower its freezing point will be. When 1 mole of a nonelectrolyte (doesn't ionize in water) dissolves in water, you get 1 mole of dissolved particles raising the b.p. and lowering the f.p. But when 1 mole of an ionic compound dissolves, like NaCl, it breaks up into 1 mole of Na+ and 1 mole of Cl-. This means you have 2 moles of particles, so the b.p. is raised twice as much, and the freezing point is lowered twice as much as if it were only 1 mole of dissolved particles.

So:

.0250 m C2H4(OH)2 gives .0250 moles of particles per 1000 g. of water (molal)
0.0200 m NaCl gives 0.0400 moles of particles per 1000 g. of water
0.00750 m MgSO4 gives 0.0150 moles of particles " " " (Mg, and SO4)
0.0100 m K2SO4 gives 0.0300 moles of particles " " " (2K's and SO4)
0.00400 m Al2(SO4)3 gives 0.0200 moles of particles " " " (5 x 0.00400)

so, the NaCl has the most moles of particles, so it raises the boiling point the MOST (highest b.p.)
and since the MgSO4 has the least moles of particles, its freezing point is lowered the LEAST (highest f.p.)


I hope this helps!

 

[Swenis]

for 3a) there are 0.015 mol of H2 (g)
b) 3.008 atm

I'm not sure about part (c) but hopefully my answers are correct and can help you solve the last part of the question! good luck! =)

Thank you! I managed to get part a, but could you explain how you found part b? I may just have a problem in my math somewhere.

 

[Swenis]

This has to do with the colligative properties of solutions. The more moles of stuff dissolved in water, the higher the solution's boiling point and lower it's freezing point will be. When 1 mole of a nonelectrolyte (doesn't ionize in water) dissolves in water, you get 1 mole of dissolved particles raising the b.p. and lowering the f.p. But when 1 mole of an ionic compound dissolves, like NaCl, it breaks up into 1 mole of Na+ and 1 mole of Cl-. This means you have 2 moles of particles, so the b.p. is raised twice as much, and the freezing point is lowered twice as much as if it were only 1 mole of dissolved particles.

So:

.0250 m C2H4(OH)2 gives .0250 moles of particles per 1000 g. of water (molal)
0.0200 m NaCl gives 0.0400 moles of particles per 1000 g. of water
0.00750 m MgSO4 gives 0.0150 moles of particles " " "
0.0100 m K2SO4 gives 0.0300 moles of particles " " "
0.00400 m Al2(SO4)3 gives 0.0200 moles of particles " " " (5 x 0.00400)

so, the NaCl has the most moles of particles, so it raises the boiling point the MOST (highest b.p.)
and since the MgSO4 has the least moles of particles, its freezing point is lowered the LEAST (highest f.p.)


I hope this helps!

This is great information! Thanks for the great explanation.

 

[Swenis]

Ok I've managed to get all of the other problems but does anyone know how to do part c for #3? Thanks, and that's the end of my questions.